Add bulk_get tests

Add an integration test suite for `_bulk_get`. Test various errors, missing
docs, bad revs, various options such as `atts_since`, `latest` and
`attachments`. Also add a test a few thousand documents and retrieve them with
bulk_get.

This is related to the effort of optimizing our bulk_get implementation [1]. In
order get some confidence we haven't broken something in the process, we're
adding a bunch of tests.

[1] https://github.com/apache/couchdb/issues/4183
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